The present invention relates, in general, to process controllers and, in particular, to a new and useful three-mode controller which processes a signal corresponding to a control variable according to a proportional plus integral plus derivative function which is adjustable using a photoresistor and variable light source.
Various processes, such as the generation of steam, require the use of controllers which adjust one or more parameters in a process such as temperature and pressure.
Three mode analog controllers are known which are designated proportional-plus-integral-plus-derivative controllers or PID controllers that obtain an integral value for a signal corresponding to a control variable, a derivative value therefor, and also a proportional value thereof. This provides a smoother regulation of the control parameter. Details of such PID controllers can be found in Steam/Its Generation and Use, 38th Ed., Babcock & Wilcox Company, 1975, Chapter 35.
As shown in FIG. 1, known three-mode controllers include three potentiometers designated 10, 12, and 14.
Potentiometer 10 is included in an integration circuit comprising capacitor C1 and resistor R1 with a movable tap for adjusting the time constant of integration. This circuit is known as a reset adjustment circuit. A control variable designated E.sub.in is applied at terminal 16. A rate adjustment for adjusting the rate of differentiation of the signal is provided by potentiometer 12 which is included in a derivative or rate circuit formed of capacitor C.sub.3 and C.sub.2 as well as variable resistor R.sub.2. For additional details, see Bailey Product Instruction 4570K11-300, published by the Bailey Controls Company of Wickliffe, Ohio.
A proportional circuit for applying a selected gain to the signal is adjusted by potentiometer 14 which includes variable resistor R.sub.3. Operational amplifiers A1 and A2 are also included in the circuitry to produce an output voltage E.sub.out at terminal 20.
The tuning adjustments of the three-mode analog controller shown in FIG. 1 have traditionally been set manually by an operator or other responsible plant personnel. Since these settings have to be manually adjusted at the controller, there is little flexibility in judiciously adapting the controller to the dynamics of the process as they occur.
There are many applications in industry which would benefit from the ability to effect a tuning modification as a direct result of actual process disturbances such as load, demand, or feed rate. Most of these opportunities are left unexplored, however, due to the lack of suitable facilities or due to the high cost and cumbersome techniques required.
FIG. 5 illustrates a super heat temperature control analog logic for a Babcock & Wilcox RB Boiler. The controller is designed to hold final superheat temperature to a preprogrammed set point value applied over a line 18 to a difference unit 22 which also receives the actual temperature from temperature transmitter 24. The difference or error signal is applied over a line 26 connected to the output of difference unit 22.
A secondary flow control loop is added to provide much faster and more suitable response. This second control loop responds to a pressure difference from pressure transmitter 28 which is processed in a controller 30 with the temperature loop information from a summing unit 32. Valve control 34 is operated according to the output of controller 30 which regulates valve 36.
A load demand signal is supplied over a line 38 to a pair of function generators 40. The demand signal is introduced to improve the overall load response. The primary temperature loop having an input at line 26 exhibits process lags (time delays) which are inversely proportional to the load demand. Consequently, higher gain and lower reset settings are more appropriate for lower load demands. The reset or integration function is provided by multiplication unit 42, function generator 40, and integrating unit 46. The gain settings are achieved by using function generators 40 and multipliers 44, and proportional unit 48.
A control system with adaptive process controllers is also disclosed in U.S. Pat. No. 3,939,328 to Davis.